Modern aircraft generally have a Synthetic Vision System called “SVS”. This system makes it possible to show the crew a synthetic image of the outside landscape generally including piloting or navigation information.
An SVS system comprises a mapping database representative of the terrain flown over, a geolocation system, electronic computation means and one or more display devices installed in the cockpit of the aircraft. The geolocation system is of the “GPS” (Global Positioning System) type. It can be coupled with the inertial system of the aircraft. The geolocation system as a whole supplies at least the following parameters: position of the aircraft in latitude, longitude and altitude and orientation of the aircraft in pitch, roll and heading.
Generally, the image is displayed on the display screens which are located on the front of the aircraft instrument panel. The image is a three-dimensional view of the outside represented as realistically as possible. The viewpoint displayed is in the axis of the aircraft.
This type of system, very attractive for the user, presents a view such that this user may be over-reassured as to the position of the elements around him or her and, in particular, the landing runways. However, depending on the quality of the positioning system, the image is not exact or may be only partially exact. The current systems start from the assumption that the quality of the positioning is sufficient to guarantee the quality of the information presented. However, this assumption may be false in certain cases, particularly when the aircraft is very close to the ground.
The types of errors that can be encountered are horizontal and vertical positioning errors which can result in setting down outside of the runway, which is unacceptable.
One means for resolving this problem is to present to the user the areas of the image which are credible and the areas which are not, given the accuracy provided by the geolocation system. The “non-credible” area is then indicated by an opaque or semi-transparent colouring or else by a deliberately blurred area. The drawback of this solution is that it can mask a large part of the synthetic terrain, whereas, in a landing runway approach phase, the terrain is generally flat around the runway. It is not therefore necessary to mask it since a horizontal and vertical positioning error is reflected by a translation or rotation of the terrain, therefore this error will be indistinct on the terrain around the runway.
Another solution is described in the U.S. Pat. No. 8,589,071 entitled “Aircraft Vision System including a runway position indicator”. It is illustrated in FIG. 1. The display comprises a specific symbol S around the assumed position of the runway P. As can be seen in FIG. 1, this symbol is larger than the real runway in order to assist the pilot in visually acquiring the landing runway by giving him or her an indication concerning the area to watch. The drawback of this solution is that it adds symbols to the graphic representation and therefore increases the workload of the pilot for interpreting all of the information displayed.